Electronic postage meters have been developed with electronic accounting and other circuitry. Postage meter systems of this type are disclosed in U.S. Pat. No. 3,978,457 for Microcomputerized Electronic Postage Meter System and in U.S. Pat. No. 3,938,095 for Computer Responsive Postage Meter. Keyboards are provided in the electronic postage meters to allow the user to enter the amount of postage to be printed. The keyboards are also used to actuate various functions in the meter such as to recall for display the amount in the ascending register, which provides a total of all postage printed by the meter, and the amount in the descending register, which is the amount of the remaining postage which may be printed.
The electronic accounting circuits of the postage meter process the information which may be entered via the keyboard. The circuits include non-volatile memory capability for storing postage accounting information which are part of the postage meter registers. Programs for the proper operation of the meter are also stored in additional memory circuits. The memory and accounting circuits have replaced the function served in previous postage meters by mechanical accounting registers.
Electronic Postage Meters, similar to mechanical postage meters have a requirement for a high level of security. This is because the postage available for printing which is stored in the meter represents postage that has been previously purchased by a user from the postal services. As a result, the information stored in the registers has a monetary value. Accordingly, the meter must be made resistant against intentional or inadvertant unauthorized changes in the registers. If, for example, an unauthorized increase occurred in the meters descending register, then postage would be available to be printed for which payment has not been made. On the other hand, if an unauthorized decrease occurred in the meter's descending register, then postage would not be available for printing for which payment has been made.
Electromagnetic radiation interference can adversely affect the electronic postage meter. This radiation includes radiation in the radio frequency spectrum, which is generally considered to range from approximately 5 kilohertz to 106 megahertz. The radio frequency spectrum encompasses some audio frequencies, television frequencies, and radar frequencies. The radiation can result in a loss or a distortion of data or programs stored in the meter. This loss or distortion may result in the improper operation of the meter including the possible improper accounting for postage which has been printed or which is available to be printed. These affects can result in the meter being rendered totally inoperative.
It has been recognized that the affects of electromagnetic radiation on electronic postage meter circuitry can be eliminated by the use of shielding. Additional shielding has been provided in some cases for the internal critical portions of the meter such as the accounting circuit. The keyboard is one area where the electromagnetic radiation can enter the system. Keyboards, however, have been designed to prevent this from occurring. These keyboards operate satisfactorily for their intended purpose without being subject to the adverse affects of electromagnetic radition. However, such keyboards are an expense to manufacture and have a large number of moving parts.
Some prior art keyboards have employed non-ferrous metal shielding with openings for the switch actuators. The switch actuators project through the openings to effectuate operation of the switches. These openings are areas where electromagnetic radiation can enter the mechanism via the keyboard unless extreme care is taken in the specific design; for example, where the openings in the shielding are too large. Moreover, although non-ferrous metal shielding provides protection against electromagnetic radiation in the gamma ray and X-ray frequency bands, such shielding is not suitable protection against the affects of electromagnetic radiation in the radio frequency band.